Field
This invention relates to surgery, specifically that involving the application of plasma to tissue.
State of the Art
"Glow discharge" phenomena are well known. The most familiar applications of such phenomena are in lighting, e.g., in fluorescent, neon, sodium and mercury lamps. Glow discharge plasmas are often described as "cold plasmas" because the energy density and wall-heating effect of such plasmas are very low. Such plasmas may also be regarded as being at thermal nonequilibrium because their gas temperatures are characteristically much lower than their "electron temperatures." The term "electron temperature" denotes a temperature (usually several thousand degrees) corresponding to the energy possessed by the electrons in a plasma. It is commonly understood that the operating conditions productive of cold plasmas are high voltage (1 - 100kV) and low pressure (usually below 10 torr). The term "cold plasma," as used in the following specification and claims, is intended to include plasmas at thermal nonequilibrium which evidence a low wall-heating effect, whether or not such plasmas exhibit the appearance and other physical characteristics normally associated with the specific cold plasma and glow discharge phenomena heretofore recognized in the art. According to this invention, cold plasmas may be produced which possess a very high energy density, for example.
As used in this specification and in the appended claims, the term "plasma" is used in its broadest context and refers to an at least partially ionized gas, which may include molecules, atoms, ions, electrons and free radicals, each moving with a velocity dependent upon its mass and its temperature. (A plasma is regarded as at thermal equilibrium only when the distribution of its particle velocities is such that the average energy of each species is approximately the same.) The average energy of a particle (e.g., an electron) can be expressed as a temperature (e.g., "electron temperature") according to the relationship ##EQU1## where m is the mass of the particle, V is the root-mean-square velocity of the particle, k is Boltzmann's constant, and T is the absolute temperature of the particle. The term "plasma" includes gases ionized to a very limited extent, e.g., 0.1 percent of its molecules, although it is often preferred to refer to such gases as being in an "energized" state. The term "energized" gas refers to any gas, whether ionized or not, which is storing energy, as a result of the application of electrical energy, in a form capable of subsequent release as heat and/or light. This term thus includes a gas which is ionized, disassociated, or in an "excited" state, including the "metastable" state. A gas is considered to be in an "excited" state when an electron of an inner orbital shell of a species (molecules, atoms, and/or ions) has absorbed a quantum of energy so that it is at a higher-than-its-ground-state energy level with respect to the nucleus; it is considered to be in the "metastable" state when an inner electron is excited to a level from which the return to ground via electro-magnetic emission is of extremely low probability. A species in the metastable state generally loses its excess energy either by imparting kinetic energy to its surroundings or by exciting other molecules, atoms or ions.
The use of D.C. arc plasma for surgery is suggested by U.S. Pat. No. 3,434,476, which discloses and claims apparatus intended for use as a surgical scalpel. The apparatus thus disclosed is apparently incapable of producing plasmas which are not substantially at thermal equilibrium. This instrument is reported to be unsafe for actual clinical use because it tends to char tissue. It is recognized in the disclosure of the aforementioned application Ser. No. 79,840 that plasmas of metastable noble gas are preferred for surgical applications. Heretofore, the advantages of electrically neutral, D.C.-induced cold plasmas for this purpose have not been suggested, however.